The slogan "espouses the idea that man created God as well as all religions, and encourages public and critical examination of the merits of religious belief," according to Indiana Atheist Bus Campaign.

The ads were launched in the windy city last week just after the Indiana Atheist Bus Campaign had similar messages up and running in South Bend, Ind., where President Barack Obama had earlier addressed graduates of the preeminent Roman Catholic university – the University of Notre Dame.

"In my opinion, this is a great message to put out there," said Reba Boyd Wooden, executive director of Center for Inquiry, an organization that promotes secularism. "Too many people think that you have to be religious to be a moral person. They don't know they have a choice. They think that it is a given that there is a god, that everyone believes there is a god, and if you don't believe in god you are a bad person. We secular humanists know that is not true."

The ads in South Bend state "You can be good without God."

Members and supporters of the Indiana Atheist Bus Campaign revealed two different motivations behind launching the ads. Some, the campaign reported, want to promote a positive message about atheism and encourage atheists to "come out" while others want to tackle religious belief head-on with science and evidence-based approaches.

According to The Chicago Tribune, the Indiana atheist group was "inspired" by a similar campaign that was launched in Europe last year by the British Humanist Association. Bus ads there read: "There's probably no God. Now stop worrying and enjoy your life."

In recent years, more atheists have come into the spotlight, loudly proclaiming their beliefs – or unbelief – and challenging Christianity. Although atheists make up a small percentage of America's population, the public nature of their efforts has not left them unnoticed, especially among Christians.

Christian scholars have responded by releasing a myriad of resources helping fellow believers to strengthen their Bible knowledge, training them in Christian apologetics to defend their faith, and in the process strengthening their own faith.